In composite video television systems such as NTSC and PAL, luminance and chrominance information share a portion of the total signal bandwidth. In NTSC, for example, chrominance information is encoded on a subcarrier of 3.579545 MHz. Within the chrominance band which extends from roughly 2.3 MHz to 4.2 MHz, both the chrominance and luminance spectra are intermingled.
A television decoder extracts both luminance spectral information and chrominance spectral information from the composite signal; however, a simple conventional television decoder cannot discern which of the higher frequency components are luminance information and which are chrominance information. As a result, these decoders generate incorrect chrominance information based upon the luminance spectrum. The term "cross color" refers to corruption of the chrominance spectrum caused by the misinterpretation of high-frequency luminance information as chrominance information. Conversely, the term "cross luminance" refers to corruption of the luminance spectrum by the misinterpretation of chrominance information as high-frequency luminance information.
Cross color is more apparent for certain images. For example, a tweed jacket of a television announcer may cause a television decoder to incorrectly produce a lot of shiny chrominance information. As another example, large vertical or substantially vertical transitions are often accompanied by bands of red and green color like a rainbow along the edge of the transitions.
Historically, it has not been very important to go to great lengths to reduce cross color because television screens were small and not very bright. But because screens are becoming larger and brighter, and because various picture enhancement techniques such as line doubling which eliminate a lot of other artifacts are becoming more common place, cross color is becoming much more noticeable and very objectionable. It has become very important to reduce cross color more efficiently. Examples of decoders in the prior art which incorporate some form of more intelligent cross color suppression schemes are set forth in U.S. Pat. Nos. 4,179,705, 4,240,105, 4,706,112, and 4,864,389.
Prior art methods reduce cross color by operating upon chrominance information encoded on the chrominance subcarrier prior to demodulation into baseband chrominance information. These methods typically incorporate cross color suppression into the decoding process, focusing on improving the separation of the chrominance and luminance information to reduce both cross color and cross luminance.
A primitive technique in the prior art teaches separating chrominance and luminance using a combination of a notch filter passing lower-frequency spectral components as luminance information and a high-frequency bandpass filter (BPF) passing higher-frequency spectral components as chrominance information. The chrominance information is subsequently quadrature-demodulated into two components such as, for example, I and Q information, or R-Y and B-Y information. The luminance and the two chrominance components are matrixed together to generate the Red, Green, and Blue (RGB) signals needed to drive conventional analog television display systems. In NTSC systems using a BPF from about 2.3 to about 4.5 MHz, all higher-frequency luminance information is passed together with the chrominance information. Cross color is very heavy.
A better approach uses comb filters to separate chrominance and luminance. The comb filter exploits the fact that, in most color television systems, luminance energy in the vicinity of the chrominance subcarrier is most probably located near harmonics of the line scanning frequency, say 15 kHz, while chrominance information is located between the luminance information. The sidebands of the luminance and chrominance information will usually overlap somewhat.
The comb filter typically has two response characteristics: one response for the luminance path and a second response for the chrominance path. This technique for separating luminance and chrominance is superior to the primitive technique described above, but it works well only when the luminance information is concentrated around harmonics of the line scanning frequency, or when the luminance information is of a vertical nature. As a result, this technique works well for vertical bars, for example, but it does not work very well for diagonal bars which cause both the chrominance and luminance spectra to spread and mix, thereby causing cross color.
Comb filter decoding of diagonal bars can be improved by filtering in the time domain using frame memory rather than filtering in the line domain using line memory. The technique is applicable to various video standards such as NTSC and PAL.
The prior art also teaches reducing cross color by averaging chrominance information across successive frames or lines using comb filters or by recirculation of frame, field, or line periods of chrominance information. As mentioned above, such techniques are generally combined with the process of decoding or separating luminance information and chrominance information prior to demodulation of the chrominance information into the baseband domain. The averaging coefficients and the recirculation coefficients are typically adapted in response to the presence of vertical transitions or motion in the television picture to prevent blurring of chrominance information in either the vertical or the temporal domains. Examples of averaging and recirculation techniques are discussed more fully in U.S. Pat. No. 4,443,817 which is hereby incorporated by reference in its entirety.
In NTSC systems, for example, the chrominance subcarrier phase rotates by 180 degrees between successive frames. This rotation causes luminance information to be misinterpreted as chrominance information which oscillates between two complementary colors such as red and green; that is, the luminance appears to be spectral energy which oscillates between two colors represented by chrominance information 180 degrees out of phase with each other. By averaging the chrominance information in two successive frames, the out-of-phase cross color information cancels thereby allowing chrominance information to be obtained which is free of cross color; however, this technique only works when the picture is stationary. When one or more objects in the picture image are moving, the amount of one-to-one correlation between luminance information in successive frames is so low that cross color suppression is difficult to do and expensive to implement. Therefore, comb filtering in the time domain does not suppress cross color very well for pictures containing objects in motion.
Cross color suppression in the prior art is difficult to accomplish in applications such as home video cassette recorders in which no stable clock or timing signal is available. In such recorders, for example, the chrominance subcarrier and the frame synchronization signal are independent; the frequencies of these two signals are allowed to vary independently. There is a need, therefore, to provide for cross color suppression without relying upon any particular clock or timing signal.
In addition, cross color suppression is very desirable in applications where only demodulated baseband chrominance information is available, especially where demodulation was performed without much regard for suppressing cross color. In such applications, for practical reasons, cross color suppression must be performed in the baseband domain.